Everyone seems  to think that they are writing OO after all they are using OO languages such as Java, Python or Ruby. But if you exam the code it is often procedural in nature.

Static Methods

Static methods are procedural in nature and they have no place in OO world. I can already hear the screams, so let me explain why, but first we need to agree that global variables and state is evil. If you agree with previous statement than for a static method to do something interesting it needs to have some arguments, otherwise it will always return a constant. Call to a staticMethod() must always return the same thing, if there is no global state. (Time and random, has global state, so that does not count and object instantiation may have different instance but the object graph will be wired the same way.)

This means that for a static method to do something interesting it needs to have arguments. But in that case I will argue that the method simply belongs on one of its arguments. Example: Math.abs(-3) should really be -3.abs(). Now that does not imply that -3 needs to be object, only that the compiler needs to do the magic on my behalf, which BTW, Ruby got right. If you have multiple arguments you should choose the argument with which method interacts the most.

But most justifications for static methods argue that they are “utility methods”. Let’s say that you want to have toCamelCase() method to convert string “my_workspace” to “myWorkspace”. Most developers will solve this as StringUtil.toCamelCase(“my_workspace”). But, again, I am going to argue that the method simply belongs to the String class and should be “my_workspace”.toCamelCase(). But we can’t extend the String class in Java, so we are stuck, but in many other OO languages you can add methods to existing classes.

In the end I am sometimes (handful of times per year) forced to write static methods due to limitation of the language. But that is a rare event since static methods are death to testability. What I do find, is that in most projects static methods are rampant.

Instance Methods

So you got rid of all of your static methods but your codes still is procedural. OO says that code and data live together. So when one looks at code one can judge how OO it is without understanding what the code does, simply by looking at the relationship of data and code.

class Database {
  // some fields declared here
  boolean isDirty(Cache cache, Object obj) {
    for (Object cachedObj : cache.getObjects) {
      if (cachedObj.equals(obj))
        return false;
    }
    return true;
  }
}

The problem here is the method may as well be static! It is in the wrong place, and you can tell this because it does not interact with any of the data in the Database, instead it interacts with the data in cache which it fetches by calling the getObjects() method. My guess is that this method belongs to one of its arguments most likely Cache. If you move it to Cache you well notice that the Cache will no longer need the getObjects() method since the for loop can access the internal state of the Cache directly. Hey, we simplified the code (moved one method, deleted one method) and we have made Demeter happy.

The funny thing about the getter methods is that it usually means that the code where the data is processed is outside of the class which has the data. In other words the code and data are not together.

class Authenticator {
  Ldap ldap;
  Cookie login(User user) {
    if (user.isSuperUser()) {
      if ( ldap.auth(user.getUser(),
             user.getPassword()) )
        return new Cookie(user.getActingAsUser());
    } else (user.isAgent) {
        return new Cookie(user.getActingAsUser());
    } else {
      if ( ldap.auth(user.getUser(),
             user.getPassword()) )
        return new Cookie(user.getUser());
    }
    return null;
  }
}

Now I don’t know if this code is well written or not, but I do know that the login() method has a very high affinity to user. It interacts with the user a lot more than it interacts with its own state. Except it does not interact with user, it uses it as a dumb storage for data. Again, code lives with data is being violated. I believe that the method should be on the object with which it interacts the most, in this case on User. So lets have a look:

class User {
  String user;
  String password;
  boolean isAgent;
  boolean isSuperUser;
  String actingAsUser;

  Cookie login(Ldap ldap) {
    if (isSuperUser) {
      if ( ldap.auth(user, password) )
        return new Cookie(actingAsUser);
    } else (isAgent) {
        return new Cookie(actingAsUser);
    } else {
      if ( ldap.auth(user, password) )
        return new Cookie(user);
    }
    return null;
  }
}

Ok we are making progress, notice how the need for all of the getters has disappeared, (and in this simplified example the need for the Authenticator class disappears) but there is still something wrong. The ifs branch on internal state of the object. My guess is that this code-base is riddled with if (user.isSuperUser()). The issue is that if you add a new flag you have to remember to change all of the ifs which are dispersed all over the code-base. Whenever I see If or switch on a flag I can almost always know that polymorphism is in order.

class User {
  String user;
  String password;

  Cookie login(Ldap ldap) {
    if ( ldap.auth(user, password) )
      return new Cookie(user);
    return null;
  }
}

class SuperUser extends User {
  String actingAsUser;

  Cookie login(Ldap ldap) {
    if ( ldap.auth(user, password) )
      return new Cookie(actingAsUser);
    return null;
  }
}

class AgentUser extends User {
  String actingAsUser;

  Cookie login(Ldap ldap) {
    return new Cookie(actingAsUser);
  }
}

Now that we took advantage of polymorphism, each different kind of user knows how to log in and we can easily add new kind of user type to the system. Also notice how the user no longer has all of the flag fields which were controlling the ifs to give the user different behavior. The ifs and flags have disappeared.

Now this begs the question: should the User know about the Ldap? There are actually two questions in there. 1) should User have a field reference to Ldap? and 2) should User have compile time dependency on Ldap?

Should User have a field reference to Ldap? The answer is no, because you may want to serialize the user to database but you don’t want to serialize the Ldap. See here.

Should User have compile time dependency on Ldap? This is more complicated, but in general the answer depends on wether or not you are planning on reusing the User on a different project, since compile time dependencies are transitive in strongly typed languages. My experience is that everyone always writes code that one day they will reuse it, but that day never comes, and when it does, usually the code is entangled in other ways anyway, so code reuse after the fact just does not happen. (developing a library is different since code reuse is an explicit goal.) My point is that a lot of people pay the price of “what if” but never get any benefit out of it. Therefore don’t worry abut it and make the User depend on Ldap.

You hear people talking about small/medium/large/unit/integration/functional/scenario tests but do most of us really know what is meant by that? Here is how I think about tests.

Unit/Small

Lets start with unit test. The best definition I can find is that it is a test which runs super-fast (under 1 ms) and when it fails you don’t need debugger to figure out what is wrong. Now this has some implications. Under 1 ms means that your test cannot do any I/O. The reason this is important is that you want to run ALL (thousands) of your unit-tests every time you modify anything, preferably on each save. My patience is two seconds max. In two seconds I want to make sure that all of my unit tests ran and nothing broke. This is a great world to be in, since if tests go red you just hit Ctrl-Z few times to undo what you have done and try again. The immediate feedback is addictive. Not needing a debugger implies that the test is localized (hence the word unit, as in single class).

The purpose of the unit-test is to check the conditional logic in your code, your ‘ifs’ and ‘loops’. This is where the majority of your bugs come from (see theory of bugs). Which is why if you do no other testing, unit tests are the best bang for your buck! Unit tests, also make sure that you have testable code. If you have unit-testable code than all other testing levels will be testable as well.

A KeyedMultiStackTest.java is what I would consider great unit test example from Testability Explorer. Notice how each test tells a story. It is not testMethodA, testMethodB, etc, rather each test is a scenario. Notice how at the beginning the test are normal operations you would expect but as you get to the bottom of the file the test become little stranger. It is because those are weird corner cases which I have discovered later. Now the funny thing about KeyedMultiStack.java is that I had to rewrite this class three times. Since I could not get it to work under all of the test cases. One of the test was always failing, until I realized that my algorithm was fundamentally flawed. By this time I had most of the project working and this is a key class for byte-code analysis process. How would you feel about ripping out something so fundamental out of your system and rewriting it from scratch? It took me two days to rewrite it until all of my test passed again. After the rewrite the overall application still worked. This is where you have an AHa! moment, when you realize just how amazing unit-tests are.

Does each class need a unit test? A qualified no. Many classes get tested indirectly when testing something else. Usually simple value objects do not have tests of their own. But don’t confuse not having tests and not having test coverage. All classes/methods should have test coverage. If you TDD, than this is automatic.

Medium/Functional

So you proved that each class works individually, but how do you know that they work together? For this we need to wire related classes together just as they would be in production and exercise some basic execution paths through it. The question here we are trying to answer is not if the ‘ifs’ and ‘loops’ work, (we have already answered that,) but whether the interfaces between classes abide by their contracts. Great example of functional test is MetricComputerTest.java. Notice how the input of each test is an inner class in the test file and the output is ClassCost.java. To get the output several classes have to collaborate together to: parse byte-codes, analyze code paths, and compute costs until the final cost numbers are asserted.

Many of the classes are tested twice. Once directly throughout unit-test as described above, and once indirectly through the functional-tests. If you would remove the unit tests I would still have high confidence that the functional tests would catch most changes which would break things, but I would have no idea where to go to look for a fix, since the mistake can be in any class involved in the execution path. The no debugger needed rule is broken here. When a functional test fails, (and there are no unit tests failing) I am forced to take out my debugger. When I find the problem, I add a unit test retroactively to my unit test to 1) prove to myself that I understand the bug and 2) prevent this bug from happening again. The retroactive unit test is the reason why the unit tests at the end of KeyedMultiStackTest.java file are “strange” for a lack of a better world. They are things which I did not think of when i wrote the unit-test, but discovered when I wrote functional tests, and through lot of hours behind debugger track down to KeyedMultiStack.java class as the culprit.

Now computing metrics is just a small part of what testability explorer does, (it also does reports, and suggestions) but those are not tested in this functional test (there are other functional tests for that). You can think of functional-tests as a set of related classes which form a cohesive functional unit for the overall application. Here are some of the functional areas in testability explorer: java byte-code parsing, java source parsing, c++ parsing, cost analysis, 3 different kinds of reports, and suggestion engine. All of these have unique set of related classes which work together and need to be tested together, but for the most part are independent.

Large/End-to-End/Scenario

We have proved that: ‘ifs’ and ‘loops’ work; and that the contracts are compatible, what else can we test? There is still one class of mistake we can make. You can wire the whole thing wrong. For example, passing in null instead of report, not configuring the location of the jar file for parsing, and so on. These are not logical bugs, but wiring bugs. Luckily, wiring bugs have this nice property that they fail consistently and usually spectacularly with an exception. Here is an example of end-to-end test: TestabilityRunnerTest.java. Notice how these tests exercises the whole application, and do not assert much. What is there to assert? We have already proven that everything works, we just want to make sure that it is wired properly.

Which one are you? I am an engineer. But maybe we should first define the differences between the two. Engineer cares about how the system is put together whereas CS cares about how it works. Do you care about the technology or the algorithm?

The Interview

Since I am on engineer, these are the kind of questions I ask in an interview:

• Imagine you are an evil developer, how do you make code hard to test?
• How would you store a math expression tree, such that you would have an evaluate() method and a toString() method which would print the tree in an infix notation placing the parenthesis only when necessary.

On the other hand a CS asks these “algorithmic” questions:

• Write a merge sort method which merges ‘n’ files into one.
• Write a method to reverse a string in place.

The solution to engineer’s question is a set of classes interacting to define a system. Whereas a solution to a CS question is a single method doing something complex. Now, I don’t know about you but when was the last time when your application consisted of a single method. When was the last time when your system broke down because someone did not know how to write one of these algorithmic methods? When was the last time when your system broke down because the interaction between classes has gone too complex? I believe that most people are asking the wrong questions in the interview.

Algorithms

Now there is a time and place when an algorithm is exactly what the doctor ordered. Video/voice codecs, image compression, revision control, database optimization, and so on. But notice that all of these are nothing without some “application” around it. Video codec is nothing without YouTube around it, and DB engine is nothing without countless utilities, and database drivers which go with it. Chances are, when you are building an application, you will fail because  your class interaction will go out of hand, not because you did not know how to solve some algorithmic problem. Libraries have already been written for the most (80%) of the algorithmic problems out there. So why do you insist that an interview candidate knows how to sort integers. Is that what he/she will be doing all day long? While building your web application?

Engineer vs CS

The thing is you need both, and preferably in one person. It is also true that no one is 100% engineer or 100% CS, there is a blend in all of us. Algorithms require raw brain power, whereas engineering requires prior knowledge (best practices) which you cannot get by thinking about the problem harder. You can only get “engineering” knowledge by learning from others. I have seen too many people focusing too much on brain power alone. We hire young kids and we let them loose on a code-base, and than we wonder why we can’t change anything because everything is tightly coupled. Hint, it is not because they are stupid.

The Reality

The truth is in the ten years or so I have been out of school I have been fighting bad code every step of the way. I can’t remember when was the last time I had to solve an algorithmic problem. The most complex/algorithmic intensive code I have written was Testability Explorer, which is a byte code analysis engine which tries to determine how testable your code is. I have written it in my free time, and the thing which enabled me to write such a complex “algorithm” were unit tests, and lots of them! I had to rely on “best-engineering-practices” to build something which is “algorithm intensive”. In other words it is my engineering background not my CS which made it possible. I think CS, is required but insufficient.

Going Forward

I would love for the industry to reflect on itself and realize that its problems is not lack of CS majors, or a the lack of bright people, but a lack of good old fashion engineers, thetn maybe we may start asking a lot more questions, to which the answer is not a single method, but a collection of classes interacting together. It took a handful of scientist to come up with an atom bomb, but it took countless of engineers to work out everything else, and software is no different. You need a handful of CS, and an army of engineers.

Take your average developer and ask “do you know language/technology X?” None of us will feel any shame in admitting that we do not know X. After all there are so many languages, frameworks and technologies, how could you know them all? But what if X is writing testable code? Somehow we have trouble answering the question “do you know how to write tests?” Everyone says yes, whether or not  we actually know it. It is as if there is some shame in admitting that you don’t know how to write tests.

Now I am not suggesting that people knowingly lie here, it is just that they think there is nothing to it. We think: I know how to write code, I think my code is pretty good, therefore my code is testable!

I personally think that we would do a lot better if we would recognize testability as a skill in its own right. And as such skills are not innate and take years of practice to develop. We could than treat it as any other skill and freely admit that we don’t know it. We could than do something about it. We could offer classes, or other materials to grow our developers, but instead we treat it like breathing. We think that any developer can write testable code.

It took me two years of writing tests first, where I had as much tests as production code, before I started to understand what is the difference between testable and hard to test code. Ask yourself, how long have you been writing tests? What percentage of the code you write is tests?

Here is a question which you can ask to prove my point: “How do you write hard to test code?” I like to ask this question in interviews and most of the time I get silence. Sometimes I get people to say, make things private. Well if visibility is your only problem, I have a RegExp for you which will solve all of your problems. The truth is a lot more complicated, the code is hard to test doe to its structure, not doe to its naming conventions  or visibility. Do you know the answer?

We all start at the same place. When I first heard about testing I immediately thought about writing a framework which will pretend to be a user so that I can put the app through its paces. It is only natural to thing this way. This kind of tests are called end-to-end-tests (or scenario or large tests), and they should be the last kind of tests which you write not the first thing you think of. End-to-end-tests are great for locating wiring bugs but are pretty bad at locating logical bugs. And most of your mistakes are in logical bugs, those are the hard ones to find. I find it a bit amusing that to fight buggy code we write even more complex framework which will pretends to be the user, so now we have even more code to test.

Everyone is in search of some magic test framework, technology, the know-how, which will solve the testing woes. Well I have news for you: there is no such thing. The secret in tests is in writing testable code, not in knowing some magic on testing side. And it certainly is not in some company which will sell you some test automation framework. Let me make this super clear: The secret in testing is in writing testable-code! You need to go after your developers not your test-organization.

Now lets think about this. Most organizations have developers which write code and than a test organization to test it. So let me make sure I understand. There is a group of people which write untestable code and a group which desperately tries to put tests around the untestable code. (Oh and test-group is not allowed to change the production code.) The developers are where the mistakes are made, and testers are the ones who feel the pain. Do you think that the developers have any incentive to change their behavior if they don’t feel the pain of their mistakes? Can the test-organization be effective if they can’t change the production code?

It is so easy to hide behind a “framework” which needs to be built/bought and things will be better. But the root cause is the untestable code, and until we learn to admit that we don’t know how to write testable code, nothing is going to change…

Reader asks:

have a question regarding testing of Active Record style domain models

The problem is: client code of these objects will instantiate them directly, modify some attributes and then invoke a save(), or insert() method directly on the object. Writing a unit test that doesnt talk to the database is difficult since the save/insert methods are inherited from a super class which handles the persistence using an object responsible for database access.

As I see it, the problem is that Active Record stlye domains cross the boundary between what you desribed as Newable/Injectable. The “model” part is newable, but the data access part is injectable (ideally).

The challenge I’ve had testing is finding a suitable point at which to mock (or stub) the database access. I thought of replacing the new CustomerModel() (for example) with a call to a factory that would be provided using dependency injection, such as factory.createCustomerModel() but this feels like a bit of hack, and indeed you’ve warned against this directly in some of your posts.

The other option would be to define something static on the model super class to allow the default database object to be replaced before the test runs the client code. Again though, this doesnt “feel” that right to me.

What would you suggest?

I am assuming that you have read: http://misko.hevery.com/2009/05/05/the-problem-with-active-record/. The issue here is that the Record should be newable (http://misko.hevery.com/2008/09/30/to-new-or-not-to-new/), and as such it should have no reference to service objects. In other words the save method needs to move some place else. A good place for it is to move to is a repository. I really like the Hibernate model as it makes the repository a “watcher” which watches your newable objects and than saves them automatically when the transaction is up.

Here is an example:

CarRepository carRepo = ... inject me...;
Car car = new Car();
car.setAbc(...);
carRepo.save(car);

No problems here. In your test just inject an InMemoryCarRepository. The cool thing is that most of your business code should only deal with car. Only very few places need to know about CarRepository. Here is something else which is cool.

CarRepository carRepo = ... inject me...;
Car car = carRepo.getById(123);

enter business logic...
car.setAbc(...);
exit business logic...

carRepo.commit();

Since CaRepository knows about all of the Cars it has given out calling carRepo.commit() automatically saves all of the cars. Since most of your app modifies the entities most of it does not even need to know about CarRepository. Only classes which need to create or delete a car need to have a reference to a CarRepository.

From testing point of view this is great since most of my unit-test will be testing code from “enter business logic…” until “exit business logic…”, which means most of my tests will not even deal with CarRepository but only have to deal with a Car.

People often ask how can I justify two people when one will do. Will this not, just double my cost? To answer this question I think it is important to discuss what pair-programing is not.

How much of your time do you spend writing code? If you answer 100% than you are lying! Between checking your emails, going to meetings, talking to people, mentoring others, finding bugs and thinking about fixing them, coding is something we do surprisingly little of. I only do coding about 10% of time, but i am probably at an extrema, so lets say you code 25% of time. Even when you are coding, you spend a lot of time testing what you have just coded, and realizing what you have just done does not work. Your end product may be 100 lines of code, but it may take you weeks to get there.

Out of 25% of coding how much do I pair? Not enough, but lets say 25% of the time. Overall you are only talking about little over 10% of my overall time I am pairing which you can say that I only cost 10% more (if you can apply accounting logic to coding). But what does this extra 10% of “cost” buy me?

At most places I have been at when a new developer shows up, they hand him a computer and say, “OK, get up-to speed and build this feature X.” Then most places do not expect anything out of the developer for few months while he “gets-up-to-speed.” Few months? I expect my developers to check in a feature the first week! But you can’t just demand it you have to approach it differently.

Let me tell you about Arthur, the last developer which joined us for a month, and how productive he was. Arthur showed up on Monday at 9am. I sat down with him behind his computer and said, “Let’s get you up-to-speed.” I made sure that he was driving, and I was talking and we started installing all of the software which he needed to develop code. The compilers, the database the IDE, checking out the code from the repository and running it. The key here is that Arthur was doing all of the work and I was just telling him what to do. By noon, he was up and running, thanks to a very intensive hand-holding from me. We went to lunch and talked about the application, not in general terms but in specific terms which Arthur could translate to what he did few minutes ego.

After lunch, I said let’s go and fix a bug. I picked a simple bug and for the most part again i was doing most of the talking but made sure that Arthur had the keyboard and understood what, and why he was doing things. By 3 pm we had a fix. We ran all of the tests, and submitted the code. We watched the build to make sure we did not break anything.

This is pairing, and it is exhausting! By 3 pm I was beat and went home, but Arthur stayed to play some more with the code. Turns out he fixed another bug on his own before he went home. Not bad for first day. Now I could have fixed the bugs myself in 1 hour flat both of them, but then Arthur would not learn anything. I sacrificed my productivity to make Arthur productive in a single day. If I did not it would take Arthur weeks before he would figure out how to set everything up how things worked and enough courage to fix a bug. Yet that is exactly what most companies do. Think about the confidence Arthur had on day one working with us. He was up and ready and he fixed two bugs on day one. WOW!

More importantly is what we did not do. We did not get distracted by checking email. How can you, you have a single monitor, and that means that Arthur would read my email. Arthur did not waste time looking for documentation which is poor or does not exist, he was spoon fed the knowledge in the most efficient way, and he did it all by himself, i was just talking, he was typing. It is amazing how much more focused you are when you have two people working on something.

Next day, we paired some more, but this time we started to build a new feature. Again I was taking Arthur through the implementation making sure that he did all of the work. By the end of the week Arthur has implemented several features and the amount of pairing we did has dropped of significantly, but instead Arthur started pairing with other engineers which had the know how which he needed to get the features done.

At the simplest level pairing is helping people one-on-one, not through emails, let me show you behind a keyboard, instead of telling you in abstract. The amount of pairing and keeping each other on track depends on many things, but you will find that more you pair the more you will learn from each other and the better the code becomes.

But Arthur was a new engineer coming up-to speed, what about existing engineers? Well let me tell you story of Pepper the senior tech lead on a project. Pepper came to me and said, Misko, I don’t like the way we deal with X in our app, what could we do to make it better. Now here is a case where Pepper has all of the know how of his app, and I know next to nothing. So we sat behind the code and I started writing what I would imagine an ideal app would look like, knowing very little about the code. Pepper had to explain why my view was wrong or too simplistic and in the process we would learn from each other and slowly converged on an ideal design, which was neither his nor my idea wholly but a collaboration of our ideas. We started refactoring, sometimes I would drive and sometimes he would. We would take turns painting ourselves in the corner and then backing out. Many times Pepper would work without me, as the refactoring was tedious, but straight forward and two of us were not needed. All together it took us 2 weeks to finish this refactoring. Pepper did the lions share of the work, but we spent about 3 full days behind a single computer together. That is what is pairing.

If you would ask Pepper, he would tell you that pairing helped him focus. He would say that without the pair he would not have had many of the ideas which we ended up having. Most importantly there are now two people which really understand the details of the design. When I or Pepper will pair with someone else, we will share these new ideas with the new developers. As team pairs with each other ideas and knowledge multiply and flow making sure that the code-base remains consistent no matter which place you look. It is as if a single person has written it. This is a great thing which helps with maintenance.

Pairing does not mean that two people always do everything together and checking each others work, that would be boring and a waste of time. Pairing means that people bonce ideas from each other and have discussions, not in abstract, but behind a keyboard, keep each other focused and on track. Result of which is code which is better than the sum of its parts. Pairing is about sharing the knowledge with your team mates so that everyone is on the same page. We will go further if we make sure that all of us pull in the same direction, something which is rare in the industry at large.